Once the seeds have germinated, you "prick them out" when there are four leaves showing . . . well spaced out in seed trays. Size varies according to the variety - planting them out is more a matter of the outdoor temperature. Once things have warmed up, April hopefully, start putting them outside during the day, then back in the warm overnight. This "hardens them off" ready for planting out about a week later.

Hope that helps.

Have a look on each packet for the planting instructions some will need to go straight into the ground such as California poppies sun flowers also the hardy the annual the earlier they can go out I have usually got all mine out by the middle of May

I have found that a catalogue like Stokes Seeds is helpful. It has the professional in mind and gives the approximate time needed to grow to transplant/sales size as well as germination times. That way you can use the expected last frost date, minus growth times, minus germination times and figure out about when you should start the seeds. (It's a Canadian/US seed company)

In Scotland it is often June before the annuals can be planted out safely

thanks for all you help